This application is based on German Application DE 198 47 008.8, filed Oct. 13, 1998, which disclosure is incorporated herein by reference.
The invention provides a nitrogen oxide storage catalyst which contains at least one finely divided catalyst material and also at least one nitrogen oxide storage component from the group of alkali and alkaline earth metals.
Nitrogen oxide storage catalysts are used for the exhaust gas treatment of lean-mix operated gasoline engines (so called lean-mix engines) and diesel engines. These engines are operated with greater than the stoichiometric air to fuel ratio, that is the oxygen content in this mixture is substantially larger than would be required for complete combustion of the fuel. The oxygen excess in the exhaust gas from these engines is also correspondingly high. For this reason the hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in the exhaust gas cannot be converted with well-known three-way catalytic converters since these require a stoichiometrically composed exhaust gas for the simultaneous conversion of these harmful substances.
However, hydrocarbons and carbon monoxide can easily be converted into carbon dioxide and water by oxidation in the presence of an exhaust gas catalyst due to the high oxygen content of these engines. The conversion of nitrogen oxides on the other hand presents great difficulties. Nitrogen oxide storage catalysts have been suggested for solving this problem. These catalysts contain basic compounds that are able to absorb the nitrogen oxides from the lean-mix exhaust gas or to react with them to give nitrates and they are thus removed from the exhaust gas. Suitable compounds for this purpose are the oxides of alkali and alkaline earth metals. Some of these compounds are present in air in the form of carbonates and hydroxides which are also suitable for the storage of nitrogen oxides.
The mode of operation of nitrogen oxide storage catalysts is described in detail in the SAE-document SAE 950809. In addition to basic storage compounds, these catalysts also contain platinum as a catalytically active component in order to oxidize nitrogen oxides, most of which are present as nitrogen monoxide, to nitrogen dioxide so that they can react with the basic storage compounds in the required manner. The storage capacity of the storage compounds reduces as storage of the nitrogen oxides increases. The storage compounds must therefore be regenerated from time to time. For this purpose, the air to fuel mixture and thus also the exhaust gas is enriched for a short period. Under the reducing exhaust gas conditions which then exist, the stored nitrates are decomposed again to give nitrogen oxides and are converted to nitrogen, water and carbon dioxide with consumption of the reducing constituents in the exhaust gas on the catalytically active component.
Nitrogen oxide storage catalysts are generally deposited in the form of a coating on the walls of the flow channels of inert support structures in a honeycomb shape. These so-called honeycomb structures are generally shaped in the form of a cylinder. They have flow channels for the exhaust gas which are parallel to the axis and these are arranged in a regular array over the cross section of the honeycomb structure. The number of flow channels per cross section of area on the honeycomb structure, also known as the cell density, is between 10 and 200 cm2. The amount of catalyst coating on the honeycomb structure, that is the loading of the honeycomb structure with the storage catalyst, is generally quoted as a concentration in grams per liter volume (g/l) of the honeycomb structure.
The basic problem with known nitrogen oxide storage catalysts is their low resistance to aging, that is their storage capacity is irreversibly damaged with increasing operating time due to the high exhaust gas temperatures. The reasons for this damage are many and various and depend on the particular formulation of the storage catalyst.
The storage compounds are generally applied in highly dispersed form to the surface of a support material in order to ensure sufficient interaction of the storage compounds with the exhaust gas. One basic aging mechanism, according to SAE Technical Paper 970746, consists of the storage compound reacting with the support material. Thus, in the case of a storage material consisting of barium oxide on zirconium oxide which has been aged for a period of 24 hours at 750xc2x0 C., the production of barium zirconate BaZrO3 has been observed. Barium oxide on titanium oxide leads to the production of barium titanate. In both cases this reaction of the storage compound with the support material was associated with a high loss of nitrogen oxide storage capacity. Zirconium oxide and titanium oxide are thus not suitable as supports for alkali and alkaline earth metal storage compounds due to their high tendency to react with barium oxide if they are subjected to high thermal stresses under the conditions of use. Aluminum oxide behaves slightly better as a support material. However even here the production of barium aluminate takes place at high temperatures with prolonged aging.
Various combinations of storage compounds and support materials which are also intended to solve the aging problem have been disclosed in the patent literature. Thus EP 0 562 516 A1 describes a catalyst of barium oxide, lanthanum oxide and platinum on a support material of aluminum oxide, zeolite, zirconium oxide, aluminum silicate or silicon dioxide, wherein at least some of the barium oxide and the lanthanum oxide form a mixed oxide. Due to this mixed oxide, the production of lanthanum aluminate, which would otherwise lead to aging of the catalyst, is intended to be suppressed. Loading concentrations of 0.05 to 10.0 mol/l are cited for the storage compounds on the honeycomb structure used as a support structure for the catalyst coating. In the case of barium oxide as a storage compound, this means a maximum loading with up to 1534 g/l. The concentrations mentioned in the examples are 0.15 mol barium oxide per liter of honeycomb structure, that is 23 g/l.
To suppress reaction of the storage compound with an aluminum oxide support, EP 0 645 173 A2 suggests dissolving lithium in the support in such a way that a solid solution of aluminum oxide and lithium is produced. 0.3 mol of barium oxide per liter of honeycomb structure is mentioned as the loading concentration in the examples, that is 46 g/l.
EP 0 653 238 A1 suggests titanium oxide, which contains at least one element from the group of alkali metals, alkaline earth metals and rare earth metals in the form of a solid solution, as support material. This document mentions 0.1 mol/l as a loading concentration for the storage compounds and thus lies within the scope of the values mentioned above.
EP 0 657 204 A1 mentions the mixed oxides TiO2xe2x80x94Al2O3, ZrO2xe2x80x94Al2O3 and SiO2xe2x80x94Al2O3 as support materials for nitrogen oxide storage catalysts. In addition, mixed oxides of TiO2, Al2O3 with alkaline earth metals and rare earth metals, in particular TiO2xe2x80x94Al2O3xe2x80x94Sc2O3, TiO2xe2x80x94Al2O3xe2x80x94Y2O3, TiO2xe2x80x94Al2O3xe2x80x94La2O3 and TiO2xe2x80x94Al2O3xe2x80x94Nd2O3, are mentioned as support materials. The loading concentrations for the storage compounds mentioned in the examples are also 0.3 mol/l.
EP 0 666 103 A1 describes a catalyst which contains a nitrogen oxide storage compound and a noble metal on a porous support material. Aluminum oxide, zeolite, zirconium oxide, aluminum silicate and silicon dioxide are suggested as support materials. The nitrogen oxide storage compound and noble metal are deposited in close association on the same support particles. In addition, the catalyst may also contain cerium oxide as an oxygen storing compound, wherein cerium oxide is kept separate from the noble metal and thus also from the nitrogen oxide storage compound. The loading concentration for the storage compounds in the examples in this document is again 0.3 mol/l.
EP 0 718 028 A1 discloses a heat-resistant nitrogen oxide storage material. The high heat-resistance is obtained by dispersing the nitrogen oxide storage compound very finely in the support material. For this purpose, a solution of a compound of at least an alkali metal, an alkaline earth metal and a rare earth metal is mixed with a solution of an oxide sol of at least one metal from the groups IIIb, IVa and IVb of the periodic system and converted into a gel, dried and calcined. The resulting storage material is amorphous. In the examples, this storage material is combined, inter alia, with a catalyst powder which contains platinum on a high surface area cerium/zirconium mixed oxide. The cerium/zirconium mixed oxide thus forms the support material here for the platinum component.
EP 0 771 584 A1 also describes a heat-resistant support material for catalysts which also consists of an amorphous mixed oxide. The amorphous mixed oxide is composed of a nitrogen oxide storage compound from the group of alkali metals, alkaline earth metals, rare earth metals and of aluminum oxide and at least one oxide from the group titanium oxide, zirconium oxide and silicon-dioxide. Aluminum oxide is an important constituent of the amorphous mixed oxide and is present in a molar ratio of 4 to 12 with respect to the storage compound. The support material may also contain cerium oxide as an oxygen storing material. The cerium oxide and nitrogen oxide storage compound should only be present in a molar ratio with respect to each other in the support material of between 0.5 and 3. Outside these limits, the heat-resistance is impaired according to data from EP 0 771 584 A1.
WO 97/02886 describes a nitrogen oxide storage catalyst in which the storage compound and catalytically active component are spatially separated from each other but are located in adjacent regions. For this purpose the storage compound and catalytic component are applied to the support structure in two superimposed layers. Alternatively, the storage compound and catalytic component may be deposited onto different support particles which are then applied together in the form of a coating on the support structure. As an alternative to this, according to this document, there is also the possibility of introducing the storage compound as a solid, relatively coarse powder material in the coating, wherein at least 90% of the powder particles have diameters in the range between 5 and 15 xcexcm. Metal oxides, metal hydroxides, metal carbonates and mixed metal oxides are described as storage compounds. The metals may be lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium or barium.
The loading concentrations mentioned for the storage compounds are 0.05 to 3 g/in3, that is 3 to 183 g/l. In the examples, honeycomb structures were coated with about 60 g/l of powdered strontium oxide.
According to WO 97/02886, the storage material may contain a sulfur absorbing component to protect it against poisoning by sulfur, preferably cerium oxide. This cerium oxide may be present in the form of particles alongside particles of the storage material or they may be dispersed in the nitrogen oxide storage compound.
EP 0 692 302 B1 discloses a process for the treatment of exhaust gas in which the exhaust gas is brought into contact with a catalyst which contains a porous support material on which platinum group metals and storage compounds are deposited. The storage compounds have an average particle diameter in the range between 0.1 and 20 xcexcm. The optimum particle diameters mentioned are 0.5 to 10 xcexcm. The storage compounds are added as powdered materials to the coating dispersion for the storage catalyst. So that their particulate structure is retained in the aqueous coating dispersion, storage compounds which are insoluble in water, from the group barium carbonate, barium oxalate and barium oleate, are used. If a dispersion in an organic medium is used, then barium acetate, barium formate, barium citrate, barium oxide, barium nitrite, barium nitrate, barium hydroxide or barium tartrate may also be used. The loading concentration on a honeycomb structure using these storage compounds is again cited as 0.3 mol/l. Due to the particle size of more than 0.1 xcexcm, undesired reaction of the storage compound with sulfur oxides contained in the exhaust gas is suppressed.
The storage capacity of nitrogen oxide storage catalysts known from the prior art is still inadequate over a long operating period. Aging may be caused, for example, by a reaction of the storage compounds with the support materials which leads to a loss of basicity and thus to storage capacity. Even when there is no tendency for reaction between the storage compound and the support material, there may still be a reduction in storage capacity due to a decrease in the specific surface area of the storage material.
Another aging process was observed, by the inventors, with storage catalysts in which, for example, the platinum therein came into direct contact with barium oxide. This is always the case when barium oxide is introduced by impregnating the catalyst with a soluble pre-cursor of barium oxide. As demonstrated by ongoing investigations, these storage catalysts exhibit an exceptional freshly prepared activity. However, if the catalyst material is loaded too heavily, the catalytic activity of the platinum for the conversion of nitrogen monoxide to nitrogen dioxide under oxidizing conditions is impaired, in particular after aging of the catalyst. This observation is probably based on a reaction between platinum and barium oxide. In the case of a nitrogen oxide storage catalyst applied to a honeycomb structure in the form of a coating, therefore, the subsequent loading of the catalyst with the storage compounds by impregnation should be restricted to less than 20 g/l.
A fourth, but largely reversible, aging process is the reaction of the storage compounds with the sulfur oxides contained in the exhaust gas to give relatively thermally stable sulfates. Sulfates can be decomposed at exhaust gas temperatures higher than 550xc2x0 C. and with stoichiometric or rich-mix exhaust gas compositions with the release of sulfur oxides and the formation of the storage compounds again.
As a result of the aging mechanisms described above, the storage capacity of storage catalysts decreases with increasing operating time and eventually falls below the storage capacity required for problem-free operation.
One solution to this problem might comprise increasing the loading of the storage catalyst with the storage compounds in order to extend the operating period within which the storage capacity falls to a value which is unacceptable for problem-free functioning of the storage catalyst. However in the case of supported storage compounds there is the problem that the current honeycomb structures can be loaded only with loading concentrations of up to 400 g/l at an acceptable cost. Higher loadings can be achieved only with costly multiple coating procedures and always involve the risk of blocking the flow channels.
About half the coating concentration available is taken up by the catalyst material in order to ensure adequate conversion rates for nitrogen monoxide to nitrogen dioxide. Since in the case of supported storage compounds the storage compounds themselves make up only about 20 wt. % of the support material, the concentration of storage compounds on the honeycomb structures which can be achieved in this way is restricted to about 20 to 40 g/l.
Subsequent impregnation of the final coating with precursor compounds of additional storage compounds can also be used only to a restricted extent since, in accordance with the aging process discussed above, there is a risk here that the catalytically active platinum metals might be restricted in their catalytic activity by the storage compounds.
Although the particulate storage compounds which are used as an alternative to supported storage compounds enable a higher amount of storage compounds to be introduced in practice into the nitrogen oxide storage catalyst, these have the disadvantage that, from the outset, they are an order of magnitude coarser than the aged storage particles on the support materials. Therefore they have only a low surface area for interaction with the exhaust gas. Their theoretical, molar storage capacity can thus be used only to a limited extent.
EP 0 303 495 B1 describes a catalyst which contains a) an active aluminum oxide, b) a stabilizer which is substantially insoluble in water, selected from the group strontium sulfate and barium sulfate, in an amount of 0.5 to 50 wt. %, with respect to the weight of active aluminum oxide, and c) a catalytically active component dispersed on the active aluminum oxide. The particles of stabilizer preferably have a size greater than 0.1 xcexcm. The catalyst is used as a three-way catalytic converter and enables simultaneous conversion of the harmful substances carbon monoxide, hydrocarbons and nitrogen oxides contained in a stoichiometrically composed exhaust gas. It is characterized by excellent thermal stability. A good thermal resistance is achieved by, for example, diffusing the barium from barium sulfate particles into the neighboring particles of active aluminum oxide and stabilizing its specific surface area at high exhaust gas temperatures of up to 1100xc2x0 C. The catalyst in accordance with this EP document is used as a three-way catalytic converter in a stoichiometrically composed exhaust gas. Operation in a lean-mix exhaust gas from lean-mix engines with alternating storage and release of the nitrogen oxides is not intended and would also fail because the barium component is consumed to stabilize the aluminum oxide particles.
DE 197 39 925 A1 describes an exhaust gas treatment catalyst for a diesel carbon black filter which reduces the combustion temperature of the carbon black. The catalyst consists of a metal oxide, at least one alkali metal sulfate selected from the group consisting of sulfates of Li, Na, K, Rb and Cs and/or at least one alkaline earth metal sulfate from the group of sulfates of Be, Mg, Ca, Sr and Ba. Alternating storage and release of the nitrogen oxides contained in the diesel exhaust gas is not intended with this diesel carbon black filter.
WO 95/09687 discloses a process for removing carbon monoxide, hydrocarbons and nitrogen oxide from oxygen-rich exhaust gases on a supported noble metal catalyst which has been pre-treated in a gas mixture of oxygen and an inert gas at temperatures above 400xc2x0 C. This catalyst has a wider temperature window for the reduction of nitrogen oxide than known catalysts. The catalyst may contain barium sulfate. No information is given about the average particle size of the barium sulfate particles. The process described converts the nitrogen oxides contained in the exhaust gas continuously, with simultaneous oxidation of carbon monoxide and hydrocarbons, to give carbon dioxide, water and nitrogen. Alternating storage and release of the nitrogen oxides does not take place.
The object of the present invention is therefore to provide a composition for a nitrogen oxide storage catalyst which enables the storage catalyst to be provided with a high concentration of storage compounds, which in the case of known storage catalysts has hitherto only been possible by introducing the storage compounds into the catalyst in the form of a solid powder with a large particle diameter and a correspondingly low interaction surface area for the exhaust gas. In particular, the object of the invention is to provide a nitrogen oxide storage catalyst which enables a substantially higher loading with storage compounds than a storage catalyst with only highly dispersed storage compounds. Another object of the invention is to provide a catalyst with improved stability of its storage capacity with respect to thermal aging during operation of the catalyst and the use of this catalyst for treating exhaust gases from lean-mix engines with alternating lean-mix and rich-mix exhaust gas compositions.
This object is achieved by a nitrogen oxide storage catalyst which contains at least one finely divided catalyst material and at least one nitrogen oxide storage component. The nitrogen oxide storage catalyst is characterized in that the nitrogen oxide storage component, after completing preparation of the catalyst, is present as finely divided barium sulfate, strontium sulfate or as a mixture or mixed crystals of the two sulfates or as their complete or incomplete decomposition product with an average particle diameter of less than 1 xcexcm.
Before describing the present invention in detail the following definitions are provided:
The freshly prepared state of the storage catalyst is the state of the catalyst after completing preparation of the catalyst, that is after completing all the production steps including any subsequent calcination.
A finely divided material is understood to be a powdered material which is introduced into the catalyst as such. In the English language patent literature the expression used for this is xe2x80x9cbulk materialxe2x80x9d or xe2x80x9cparticulate materialxe2x80x9d. These materials are frequently used as support materials for catalytically active components or other highly dispersed constituents of the catalyst. For this purpose, the support materials must have a high specific surface area, (also BET surface area, measured for example according to DIN 66132) for the adsorption of these components. In the context of this invention, the finely divided materials are called high surface area if their specific surface area is more than 10 m2/g.
Highly dispersed materials have to be differentiated from the finely divided materials. Highly dispersed materials may be deposited for example by impregnation onto finely divided, high surface area support materials. For this, the support materials are generally impregnated with water-soluble precursor compounds of the highly dispersed materials. As a result of an appropriate thermal treatment, the precursor compounds are then converted into the highly dispersed materials. The particle size of these highly dispersed materials is about 5 to 50 nm. In the case of highly dispersed barium oxide on a support material, typical particle diameters of 20 nm (0.02 xcexcm) were determined by the inventors with the aid of XRD analysis.
Catalyst materials in the context of this invention are understood to be any components of a conventional exhaust gas catalyst. A very wide variety of support materials and catalytically active components, oxygen storing materials and so called promoters are included among these. The catalytically active components and promoters are generally deposited in highly dispersed form on the support materials. Noble metals from the platinum group and of these in particular platinum, palladium, rhodium and iridium are suitable as catalytically active components for the purposes of the invention. The promoters are generally base metals which modify the catalytic activity of the noble metals.
Suitable support materials for the catalytically active components are known from the prior art and are high surface area support materials such as for example active aluminum oxide, cerium oxide, zirconium oxide, titanium oxide, silicon dioxide, zeolites and the mixed oxides aluminum silicate and cerium/zirconium mixed oxides. The support materials may be stabilized against thermal stresses encountered during the treatment of car exhaust gases by doping with, for example, lanthanum oxide.
The expression xe2x80x9cactive aluminum oxidexe2x80x9d is understood to mean high surface area aluminum oxides from the transition series of the crystallographic phases. These include chi, delta, gamma, kappa, theta and eta-aluminum oxide. The active aluminum oxides have specific surface areas of up to 400 m2/g. Gamma-aluminum oxide (xcex3-Al2O3) is preferably used. For thermal stabilization, the active aluminum oxides may contain, for example, lanthanum oxide, barium oxide or silicon dioxide.
Storage compounds in the context of this invention are elements from the alkali and alkaline earth metals. These are preferably potassium, rubidium, cesium, magnesium, calcium, strontium and barium. They produce highly basic oxides which can bond nitrogen dioxide in the form of nitrates. The oxides in the storage components are therefore also called storage compounds or active storage compounds. The expression xe2x80x9cstorage compound,xe2x80x9d however, is also understood here to mean the reaction products of the oxides with air or with the exhaust gas components to give carbonates and hydroxides which are also able to store nitrogen oxides as nitrates. The storage capacity of the storage compounds is generally greater the more basic is the compound.
The storage materials must be differentiated from the storage compounds. Storage materials are supported storage compounds, that is the storage compounds deposited onto suitable support materials in a highly dispersed form. In the context of this invention, however, storage compounds which are present in finely divided form are also called storage materials.
Decomposition products of barium and strontium sulfate are those compounds which are formed from sulfates in a reducing atmosphere during calcination. These are generally the oxides, carbonates and hydroxides of barium and strontium, that is active storage compounds.
Following this explanation of the expressions used, the invention is described in more detail in the following.